This invention generally relates to communications systems which require coherent detection of pulse or pulse compressed signals and in particular to a sample data phase locked loop that is adaptive, second order and uses baseband polar coordinate processing.
Communications systems such as the proposed USAF SEEK TALK system employ coherent doppler trackers for base line systems. The use of this type of circuit is particularly advantageous in that: (a) there is no doppler tracking loss during data detection even at highest doppler rates; (b) coherent detection is optimum which is particularly important since the modem must operate at low signal-to-noise ratios; (c) once a coherent reference is established it is relatively easy to generate a coherent signal for the array; and, (d) a frequency indication is provided which is useful for identifying the same signal in adjacent range bins.
Coherent doppler trackers can be implemented in either analog or digital form. Breakthroughs in analog/digital converter technology, large scale integration of digital circuitry and the operational stability of digital processing makes digital implementation attractive. The sample data phase locked loop of the invention is an improvement to the digital implementation.
A standard technique for coherent data detection is the Costas receiver. This receiver uses a synchronous detector and a phase locked oscillator to provide a coherent reference for the received signal. This type of receiver, however, operates with rectangular coordinate X, Y pulse components and consequently requires complex signal processing. It does not measure coherence between an estimated phase and the input signal phase and it utilizes a constant bandwidth through all modes of operation. This results in some performance degradation.
The present invention on the other hand: (a) coherently tracks a repetitive complex pulse; (b) converts the X, Y pulse components to polar coordinate R, .phi. components to simplify signal processing; (c) provides a means for monitoring the coherence between an estimated phase and the input signal phase; (d) provides an adaptive second order loop where the loop bandwidth can be adjusted by the coherence measurement. This provides wide bandwidths during initial acquisition, medium bandwidths during acquision/track transition, and narrow bandwidths during tracking; and (e) implements the sample data phase lock loop with digital processing which can be time shared between multiple receivers and which allows a low cost implementation.